The present invention relates generally to a high voltage linear FET amplifier which operates at voltage levels of up to ten's of thousand's of volts with power dissipation capabilities in the kilowatt range. It is useful, for example, in radar high voltage transmitter applications.
Previously, high voltage linear amplification was performed with vacuum tubes. Vacuum tubes could only be used for positive plate supply voltages--P-Channel vacuum tubes do not exist. The stand-off voltage of vacuum tubes is limited to about 40 KV. The vacuum tube has thermonic and shot noise not inherent in FET's. The vacuum tube requires auxiliary supplies, filament, screen grid, etc. More important, the vacuum tube generates electrical noise resulting from acoustical vibration.
In the past, there was every justification for using vacuum tubes. Only limited success was achieved by operating direct-coupled series transistors in the linear mode. The complicated biasing networks required for voltage division and signal distribution were cumbersome and impractical. When more than two or three transistors were operated in series, failures would occur because of the dynamic shift between the d-c and signal ground references. D-C coupled operation of cascade bi-polar transistors was never practical because of the base current drive requirements of the transistors. Linear operation of both bi-polar and field effect transistors operated in the linear mode was possible by using r-f carrier modulation or laser modulation to control the inputs of the series connected transistors. These methods proved overly complicated and did not provide an overall generic solution for many applications.
United States patent references of interest include U.S. Pat. No. 4,595,884 to Miller, Jr., which shows in FIG. 1 a bridge amplifier having successive operational amplifier stages 17 and 20 with respective feedback resistors 16 and 19 connected from the amplifier is taken between terminal 9 and ground while the input of the first stage is connected across Wheatstone bridge 51. Similar amplifier stage connections are shown at 150 and 160 in FIG. 4 of Tokura et al U.S. Pat. No. 4,471,315. Fabri, in U.S. Pat. No. 4,232,272, shows a high gain amplifier consisting of a plurality of cascaded stages each comprising an active component, a coupling network and a negative feedback network. In this patent, operating current is fed to the several amplifier stages in counterflow to the incoming signals so that its intensity progressively increases from the first stage to the last, as does the amplification of the signals. Page in U.S. Pat. No. 4,429,416 shows a plurality of differential amplifier stages cascaded in a directly coupled configuration. Masuda et al in U.S. Pat. No. 4,524,327 describe an operational amplifier with three cascade-connected amplifier stages.